1. Field of the Invention
This invention relates to centrifugal pumps for liquids and in particular, though not exclusively, to such pumps for use in supplying a mixture of abrasive particles in a liquid.
It is an object of the invention to provide an improved form of centrifugal pump which is particularly suitable for the application described above.
2. Summary of the Invention
Various features provided by the pump of the invention are set out below.
The pump has a casing which provides a pumping chamber of cylindrical form and an impeller of circular form is disposed in the chamber for rotation about an axis coincident with the longitudinal axis of the chamber. The chamber has an inlet and an outlet and a number of substantially radially extending vanes on the impeller draw liquid and abrasive into the chamber through the inlet and expel this mixture via the outlet. The dimensions of the impeller and pumping chamber are such that the clearance between the outer periphery of the impeller and the adjacent wall of the pumping chamber is from 1/8 to 5/8 inches and the clearance between the radially extending edges of the impeller vanes and the adjacent axially facing ends of the pumping chamber is from 1/16 to 3/8 inches.
The impeller may be mounted on a vertical shaft which extends through to the top of the pump casing by passing through a sealing passage which permits a certain amount of the liquid and abrasive from the pumping chamber to escape to discharge between the shaft and the wall of the sealing passage so that the passage is sealed against the intake of air.
The impeller may comprise a circular disc with a first series of circumferentially spaced radially extending vanes on one side of the disc and a second series of smaller circumferentially spaced radially extending vanes on the other side. The first series of vanes are arranged to do the major part of the pumping while the second set of vanes although doing some pumping are mainly arranged to generate a back pressure which opposes the tendency of the first set of vanes to pump liquid out through the sealing passage. This generation of a back pressure ensures that the sealing passage is not subjected to the full pressure present in the main part of the pumping chamber.
The impeller may comprise a load bearing metal portion and an outer layer of rubber or other rubber-like or plastics material which resists abrasion by the abrasive material. The securing of the abrasive resistant layer to the load-bearing portion may be assisted by providing the load-bearing portion with recesses or similar formations which provide a key for the outer abrasive resistant layer.
The inside of the pumping chamber may also be provided with an abrasion resistant rubber or other plastics layer as may be the walls of the sealing passage which also come in contact with the liquid and abrasive mixture.
The pump casing may be arranged to be split into two parts about a radially extending plane which is perpendicular to the longitudinal axis of the pumping chamber. By appropriate axial positioning of the plane on which the pump casing divides it is possible to arrange that the first series of impeller vanes which provide most of the pumping action do not direct the majority of the pumped fluid across the join between the casing parts. This assists in reducing turbulence inside the pumping chamber.
Also with a two part casing as described above it is possible to arrange the shape of the two parts of the casing to be such that they can be cast in a simple die which does not require the use of loose cores and which need not be split into more than two portions to enable the casing parts to be removed after casting.